The present invention relates to a projection type liquid crystal display apparatus which uses three transmission type liquid crystal display panels as, for example, a light valve.
Heretofore, for example, such a projection type liquid crystal display apparatus has been proposed wherein three transmission type liquid crystal display panels are used as the light valve. The transmission type liquid crystal display panels are illuminated by three primary color illuminating lights and transmission light images of three channels whose luminance is modulated by a liquid crystal display panel are re-combined by using a four-division dichroic prism so as to be projected by a projecting lens.
FIG. 1 shows an example of such an optical system. Referring to FIG. 1, a red modulating transmission type liquid crystal display panel 51R, a blue modulating liquid crystal display panel 51B and a green modulating liquid crystal display panel 51G are disposed. Image signals of three primary colors, that is, a red (R), a blue (B) and a green (G) are supplied to the liquid crystal display panels 51R, 51B, 51G, respectively.
Furthermore, polarizing plates 52R, 52B, 52G, each having a vertical polarization axis, are located at the outgoing sides of the liquid crystal display panels 51R, 51B, 51G, respectively, so that each outgoing light may be a vertical polarized light. The reason why the outgoing light from each liquid crystal display panel is a vertical axis is that the polarization axis of a light beam comprising the vertical axis makes the shielding character of the dichroic prism better.
For example, after the luminance of a red illuminating light 53R is modulated by the red modulating liquid crystal display panel 51R, a vertical polarized red outgoing light which passes through the polarizing plate 52R is incident on a four-division dichroic prism 54 comprising bonded glass blocks 54a, 54b, 54c and 54d. A red band alone of the incident light is selectively reflected by red light reflecting surfaces 55a, 55b formed on the glass blocks 54a, 54b, and then projected by a projecting lens 56.
Furthermore, after the luminance of a blue illuminating light 53B is modulated by the blue modulating liquid crystal display panel 51B, a vertical polarized blue outgoing light which passes through the polarizing plate 52B is incident on the four-division dichroic prism 54. A blue band alone of the incident light is selectively reflected by blue light reflecting surfaces 55c, 55d formed on the glass blocks 54c, 54d, and hence projected by the projecting lens 56.
Furthermore, after the luminance of a green illuminating light 53G is modulated by the green modulating liquid crystal display panel 51G, a vertical polarized green outgoing light which passes through the polarizing plate 52G is incident on the four-division dichroic prism 54. The incident light passes through the red light reflecting surfaces 55a, 55b, whereby a long wave length component thereof is attenuated. The incident light passes through the blue light reflecting surfaces 55c, 55d, whereby a short wave length component thereof is attenuated. As a result, the green band alone is selectively penetrated.
Thus, red, blue and green transmission image lights are composed of one another, and then the composed light is projected and displayed by the projecting lens 56. The reason that the shielding characteristic of the reflecting surface of the dichroic prism 54 is constructed to have the green band as a transmission channel is that a whole transmittance of the dichroic prism can be increased. An arrangement of the channel will be described below.
By the way, according to the projection type liquid crystal display apparatus shown in FIG. 1, the transmission characteristic of a blue light passing through the red light reflecting surfaces 55a, 55b of the dichroic prism 54 is shown by, for example, a curved line 61 in FIG. 2. Furthermore, the transmission characteristic of a red light passing through the blue light reflecting surfaces 55c, 55d is shown by a curved line 62 in FIG. 2.
When the green illuminating light having the band shown by a curved line 63 in FIG. 2 is incident on the dichroic prism 54, as shown in a hatched portion 64 in FIG. 2, among the long wave length components of the green illuminating light 63, some light components are reflected by the red light reflecting surfaces 55a, 55b having the transmission characteristic shown by the curved line 61.
The reflected lights become a leaked light from a green channel to a blue channel, and the leaked light illuminates a transistor element formed at the outgoing side of the blue modulating liquid crystal display panel 51B. Thus, a current leakage of the transistor or the like occurs, whereby a deterioration of an image quality such as a contrast or the like is caused.
Similarly, as shown by a hatched portion 65 in FIG. 2, among the short wave length components of the green illuminating light 63, the leaked light component reflected by the blue light reflecting surfaces 55c, 55d having the transmission characteristic shown by the curved line 62 illuminates the transistor at the outgoing side of the red modulating liquid crystal display panel 51R. Accordingly, the deterioration of the image quality such as the contrast or the like is caused.
On one hand, in order that the leaked light component at the long and short wave length sides of the green illuminating light may not illuminate the outgoing side of the blue and red modulating liquid crystal display panels, the following systems are proposed. That is, as shown in FIG. 3, a blue transmission filter 57B is disposed at the back of the polarizing plate 52B at the outgoing side of the blue modulating liquid crystal display panel 51B, or a red transmission filter 57R is disposed at the back of the polarizing plate 52R at the outgoing side of the red modulating liquid crystal display panel 51R. However, in any system, since the light transmittance is reduced, an amount of projected light is reduced.
Furthermore, in order that the leaked light component of the green illuminating light may not illuminate the outgoing side of the liquid crystal display panels for two other channels, the band of the green illuminating light is considerably reduced so that the leaked light component may not be generated. However, since the amount of green component light is reduced, the amount of projected light is reduced.
Furthermore, according to the above apparatus, the red light reflecting surfaces 55a, 55b are formed on different glass blocks 54a, 54b, and also the blue light reflecting surfaces 55c, 55d are formed on different glass blocks 54c, 54d. Accordingly, due to the difference between the characteristics of the reflecting surfaces, a tint of the green transmission light at the right side of a projecting screen is different from that at the left side, resulting in the deterioration of the image quality such as a color uniformity or the like.